Information storage is a necessary component of all computer systems. Typically, digital information is stored on various recording media either optically or magnetically. In the magnetic storage of digital information, the recording media includes tape, hard disk, and floppy disk.
Hard disk systems typically consist of one or more disks which are mounted on and rotated by a common spindle. Each disk contains a plurality of concentric tracks wherein serial data can be magnetically recorded for later recovery by a transducer or head positioned with respect to the desired track. Since information is typically recorded on both surfaces of each disk, each disk surface has a corresponding head, with all the heads being mounted on a common movable carriage. Thus, to obtain access to the recorded information, means must be provided for accurately positioning the carriage, and therefore the heads, over the track which contains the desired information.
One system for head positioning includes one or more disks as described above, where one surface of one disk is dedicated solely for the purpose of providing position and timing information for the servo control system of the disk drive. This surface contains a formatted servo data code which is electronically recovered by a servo head, mounted to the same carriage as the remaining data head(s). Using the dedicated servo surface and servo data code recorded thereon, the servo control system of the disk drive can move the data heads to a specific location on the data disks to write or read the desired information.
Due to the dramatic increase in data storage densities over the last few years, the need for a more efficient and fault-tolerant servo data code has become evident. Higher track densities (TPI) and seek velocities have made it necessary to increase the servo sample rate to maintain an acceptable level of seek performance. It has also been shown that higher bit densities (BPI) can have an adverse effect on the servo performance by altering the recorded servo information through adjacent pulse interaction and radial phase incoherence.
As track densities, data densities, and seek velocities have increased over the years, so have the methods and mechanisms for recovering dedicated servo information. As a result, a variety of servo data codes and servo control systems have been proposed.
One commonly used servo data code is described by di-bits which are written in multiple frames on each track of the servo surface. A di-bit is represented by a positive transition immediately followed by a negative transition. A frame is an arcuate portion of the servo surface spanning several tracks on which is recorded a sync di-bit, a code di-bit, and two or more additional di-bits. The sync di-bit is used to provide sync and timing information for the servo control system. The code di-bit is used to encode index and guard band patterns on specific zones on the disk. The additional di-bits in each frame of servo data are used to provide head position and velocity information used by the servo control system for positioning the data heads.
In such a system, sync and code di-bits are overwritten to achieve a constant amplitude from track to track. This overwriting process occurs as the writing head moves radially across the disk. Any circumferential movement of the writing head causes phase incoherence in the signal being written, producing signal degradation. This distortion ultimately affects the performance of the servo system. In addition, the presence of the sync and code di-bits and the sequence in which the di-bits are written in the prior art servo data codes result in asymmetrical interference patterns. As data densities increase, these phenomena becomes more pronounced due to narrower pulse widths and higher writing frequencies.